A reader of this blog — Jane, in fact: the only person who frequently comments on the contents and asks questions — has asked me how lightning ‘works’ as it were; what it is, what causes it, whether it’s a sufficient threat to homeland security for president Obama to order limited military intervention, etc.
On the assumption that there are other readers who don’t quite understand thunderstorms, and on the further less plausible assumption that they would like me to tell them, that they wouldn’t prefer to remain in their usual complacent state of bovine ignorance, here goes:
You probably know that lightning is electricity. A huge electric spark. Now normal household electricity has a voltage — a pressure — of about 220 volts, (half that in parts of America; homeland security again I expect) and it can, if you’re not careful, kill you. It takes a pressure of 30,000 volts to jump a one centimetre gap. So you can imagine — well no; you probably can’t — just how huge an electric spark a flash of lightning from cloud to ground (or vice-versa — some people insist it travels that way, but really it makes no difference) — must be. Any electric spark makes a noise, and a lightning flash makes the huge noise called thunder.
But how does it happen? Well, you may have noticed, when pulling off a jumper in a darkened room, some tiny little sparks and a crackling noise, especially if the jumper is made of an artificial fibre. The jumper is getting electrically charged by the friction of being pulled off. An electric charge is a surplus, or a shortage, of electrons: when you rub something, you quite literally rub electrons off its molecules. But things don’t like being electrically charged; they want to have the right number of electrons (which are negatively charged) to balance the positive charge of the molecules’ nuclei. Rub enough electrons off, and the tension created will be enough that something has to give: electrons will jump off the nearest large object (preferably one large enough not to ‘notice’, as it were, that it too now has too few electrons) to balance things out.
If you have a cat you can demonstrate this amusingly (for you, perhaps not so very funnily for the cat) by stroking it vigorously with one hand while holding a knuckle of the other hand close to, but not touching, its nose. With any luck, a tiny spark will jump between knuckle and nose. (It doesn’t really hurt the cat, just surprises it.) (I admit I haven’t tried this myself, but that’s because I don’t like cats and avoid touching the creatures.)
So: as a cloud moves through the upper atmosphere, electrons get rubbed off it. Lots and lots of electrons: enough, eventually, to give it such a huge negative charge that lightning flashes between cloud and ground to balance things out. It usually takes a rather jagged or forked route because electricity will always travel by the easiest route, and different parts of the atmosphere are more or less easy for it to travel through, according to their dampness or the presence of dust particles.
Sheet lightning as opposed to forked? Sheet lightning is just the normal forked sort happening far enough away that rather than seeing it directly, one sees it reflected from intervening clouds, often just above the horizon.
Got that? Here’s a pretty picture of lightning: